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Type:
Bug
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Resolution: Won't Fix
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Priority:
Minor - P4
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None
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Affects Version/s: 2.4.12
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Component/s: Querying
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None
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Fully Compatible
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ALL
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None
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None
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None
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None
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None
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It's technically possible to nest an $or under $elemMatch with the intent of finding array elements that match at least one criteria. However, in 2.4 the index is not used for that search. The new query engine on 2.6.5 handles the case as expected.
Here is a repro from 2.4:
db.ggg.drop()
db.ggg.insert({party:[{a:5,b:10},{a:3,b:2}]})
db.ggg.insert({party:[{a:5,b:10},{a:4,b:3}]})
db.ggg.insert({party:[{a:5,b:0},{a:4,b:3}]})
db.ggg.insert({party:[{a:1,b:0},{a:4,b:3}]})
db.ggg.insert({party:[{a:0,b:1},{a:2,b:0}]})
db.ggg.ensureIndex({"party.a":1,"party.b":1})
Output from 2.4:
> db.ggg.find({ "party": { $elemMatch: { $or: [ { "a" : 5, "b" : 10 },{ "a" : 3, "b" : 2 } ]} } }).explain()
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 2,
"nscannedObjects" : 5,
"nscanned" : 5,
"nscannedObjectsAllPlans" : 5,
"nscannedAllPlans" : 5,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
},
"server" : "AD-MAC10G.local:27017"
}
Output from 2.6 for comparison:
db.ggg.find({ "party": { $elemMatch: { $or: [ { "a" : 5, "b" : 10 },{ "a" : 3, "b" : 2 } ]} } }).explain()
{
"clauses" : [
{
"cursor" : "BtreeCursor party.a_1_party.b_1",
"isMultiKey" : true,
"n" : 2,
"nscannedObjects" : 2,
"nscanned" : 2,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
"party.a" : [
[
5,
5
]
],
"party.b" : [
[
10,
10
]
]
}
},
{
"cursor" : "BtreeCursor party.a_1_party.b_1",
"isMultiKey" : true,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0,
"indexBounds" : {
"party.a" : [
[
3,
3
]
],
"party.b" : [
[
2,
2
]
]
}
}
],
"cursor" : "QueryOptimizerCursor",
"n" : 2,
"nscannedObjects" : 3,
"nscanned" : 3,
"nscannedObjectsAllPlans" : 3,
"nscannedAllPlans" : 3,
"scanAndOrder" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"server" : "AD-MAC10G.local:37017",
"filterSet" : false
}